Head through the valley at the start, and just before the buildings ahead, find an opening between two large trees. Maneuver through the rocks and you should find the blue key. Now go towards the buildings, and turn left, going east through either hall to a blue switch. This switch opens up a door on the opposite end, so go through. Follow the path on the left to a switch which opens the bars on the right, then just follow the next path, going around the bars ahead. Take the bridge to the room with the rocket launcher, and go south from there all the way until you reach the exit switch.